(Adapted from applicant's abstract) Atherosclerosis is the most common cause of death in the United States. Elevated plasma levels of LDL are a major risk factor for the development of this disease. The major pathway by which LDL is catabolized is via the LDL-receptor (LDL-R). Therefore, factors that impede LDL-LDL-R interactions promote atherosclerosis. An unexplored question is whether platelet activation modulates LDL-R function. Whereas the role of platelets in the terminal thrombotic phase of this disease is well- established, it is less certain whether persistent platelet activation accelerates the pathogenesis of the atherosclerotic plaque. In view of the fact that certain clusters of positively charged residues on apolipoprotein Beta-100 are required for optimal binding of LDL to the LDL-R, the investigators tested the hypothesis that platelet factor 4 (PF4], an abundant lysine rich protein released upon platelet activation, can complete for receptor binding and thereby impede lipoprotein clearance and catabolism. Pilot data provided support for this hypothesis by demonstrating that PF4 binds to the LDL-R with nM affinity, inhibits the binding and degradation of LDL in vitro, and prolongs the plasma clearance of LDL in vivo. It is now proposed to study the biochemicals basis of the PF4-LDL-R interaction in greater detail and to develop models to elucidate the role of this platelet protein in the development of atherosclerosis through the following specific aims; 1) Specific Aim 1: Characterization of PF4 binding to the LDL-R and it's consequences in vitro. The binding kinetics of PF4 to cell lines that overexpress LDL-R as well as to recombinant soluble receptor will be measured using surface plasmon resonance. The effect of PF4 on the binding and cellular metabolism of LDL and apoE will be studied using cells that are genetically lacking or overexpress LDL-R and in which the level of proteoglycan expression has been controlled. Specific Aim 2: Effect of PF4 on lipoprotein metabolism and atherosclerosis in vivo. Adenoviral-mediated gene transfer of PF4 will be used to analyze changes in LDL clearance and endogenous lipoprotein levels in vivo. The propensity to develop hyperlipidemia and atherosclerosis will be examined in transgenic mice that overexpress human PF4. These studies are designed to gain insight into a novel mechanism by which persistent platelet activation may promote the development of atherothrombotic disease. An understanding of the structural basis of the PF4-LDL-R interaction may identify a potential locus for therapeutic intervention. This research proposal is part of comprehensive training program designed to prepare the applicant for a career as an independent investigator in the field of vascular biology.